More than 8.5 million people in the UK had high blood pressure in 2008. Photograph: Alamy

The health watchdog is planning a major overhaul of how the NHS deals with cases of high blood pressure in a move that would see patients sent home from surgeries with a monitor for 24 hours.

The concern is that those suspected of having the condition are being prescribed medication based on readings taken by the GP which may be influenced by waiting room nerves.

Draft guidelines from the National Institute for Health and Clinical Excellence (Nice), which are open to consultation, propose changes that could see a cut in the number of people under 40 diagnosed with high blood pressure, also known as hypertension thanks to more accurate testing.

However, the guidelines also urge doctors to ensure they look at the lifetime possibility of heart attack or stroke when deciding to exclude a person from treatment.

While more than 8.5 million people in the UK have been diagnosed with hypertension, medical experts say millions are being misdiagnosed often as a result of "white coat" syndrome or waiting room nerves.

As many as 32% of men and 29% of women in England in 2008 had high blood pressure (defined as a systolic pressure of 140mmHg or higher, or a diastolic pressure of 90mmHg or higher) or were being treated for the condition.

The Nice guidelines stipulate that if blood pressure measurements taken during a consultation are 140/90mmHg or higher then extra confirmation should be obtained with either ambulatory blood pressure monitoring (ABPM), or home blood pressure monitoring (HBPM).

Ambulatory testing involves sending people home with a kit to measure their blood pressure over 24 hours. Clinicians can then download this information to get a more accurate picture of a patient's true blood pressure reading.

This will prevent people receiving treatment when they do not need it. Blood pressure in a clinic can often rise owing to "white coat" syndrome, where patients feel anxious in front of a doctor and their blood pressure goes up.

Bryan Williams, professor of medicine at the University of Leicester, who led the development of the guidelines, said doctors currently have to bring people back to the clinic twice or more to get an accurate reading.

"As many as 25% of young people might record having high blood pressure when they go to the doctor when they actually don't have it.

"This is much more likely to happen when you are younger. When you are older, you are more likely to be hypertensive but, if you are at the margins and particularly if you are younger, then there's a big error rate with diagnosis.

"We don't want young people not to get treatment but the evidence shows that if you used ambulatory testing before you treat, you would get the correct diagnosis more often."

He said the guidance was not just cost-effective, but was cost-saving in the long run as less cash is spent on treatments.

However, Williams added that the overall number of people treated in the UK would not change significantly as a direct result of the guidelines. High blood pressure, which can result in part from factors such as obesity, poor diet, lack of exercise or family history, can lead to stroke, heart attack, heart failure, chronic kidney disease and cognitive decline.

About nine out of every 10 people with high blood pressure are said to have primary hypertension, when there is no major identifiable cause.